UNIT 8 – PLANTS

CHAPTER 29 – PLANT DIVERSITY

Good review game at the end of ch. 30: http://www.quia.com/rr/31120.html YOU MUST KNOW:

Why land plants are thought to have evolved from green algae. Some of the disadvantages and advantages of life on land. That plants have a unique life cycle termed alternation of

generations with a gametophyte generation and a sporophyte generation.

The role of antheridia and archegonia in gametophytes. The major characteristics of bryophytes The major characteristics of seedless vascular plants.

Use these with plant reproduction:Here are the links:

Part 1 - focuses on flower structure and the evolutionary importance of self-fertilization vs. cross-fertilization strategies:http://www.youtube.com/watch?v=OBi2iHIJIkg

Part 2 - focuses on the interaction between angiosperms and pollinators:http://www.youtube.com/watch?v=xEm7eJDdgJQ

Part 3 - focuses on the angiosperm life cycle and cell communication during pollen tube growth:http://www.youtube.com/watch?v=ANTGe_8vP4A

Part 4 - focuses on double fertilization, embryo development, the role of fruits, and seed germination:http://www.youtube.com/watch?v=uiGfbeccDwg

Part 5 - focuses on the similarities and differences in plant vs. animal development and asexual reproduction in plants:http://www.youtube.com/watch?v=mZ1DxYrIPKI

OBJECTIVE QUESTIONS:An Overview of Land Plant Evolution:

1. Describe four shared derived homologies that link charophyceans and land plants.

2. Describe five characteristics that distinguish land plants from charophycean algae. Explain how these features are adaptive for life on land.

3. Define and distinguish among the stages of the alternation of generations life cycle.

Bryophytes:4. List and distinguish among the three phyla of bryophytes. Briefly describe the characteristics of each group.5. Explain why bryophyte rhizoids are not considered roots.6. Diagram the life cycle of a bryophyte. Label the gametophyte and sporophyte stages and the locations of gamete production, fertilization, and spore production.7. Describe the ecological and economic significance of bryophytes.

The Origin and Diversity of Vascular Plants8. Describe the five traits that characterize modern vascular plants. Explain how these characteristics have contributed to their success on land.9. Distinguish between the homosporous and heterosporous condition.10. Explain why seedless vascular plants are most commonly found in damp habitats.11. Describe the production and dispersal of fern spores.

NOTES:

I. LAND PLANTS EVOLVED FROM GREEN ALGAE Land plants evolved from a group of green algae called charophyceans.

There are many key traits that land plants share with their ancestors:a. Rose-shaped protein complexes for cellulose synthesis –

these protein complexes are found in the cell membrane of both groups of organisms. Their cell walls also contain a higher percentage of cellulose that other green algae.

b. Peroxisome enzymes – these cell organelles help to minimize the loss of organic material due to photorespiration

c. Structure of flagellated sperm d. Formation of a cell plate – The formation of the cell plate during

cell division involves the formation of an alignment of Golgi-derived vesicles and cytoskeletal elements in both groups.

e. Genetic evidence – comparison of nuclear and chloroplast genes also show common ancestry.

II. CHARACTERISTICS OF THE PLANT KINGDOM Land plants also have the following five traits that are absent in green

algae:a. apical meristems – embryonic plant tissue in the tips of

roots and shoots that supply new cells for growth

b. alternation of generations – a life cycle in which there is both a multicellular diploid form (sporophyte) and a multicellular haploid form (gametophyte). The gametophyte generation is named for its production of haploid gametes (eggs and sperm) by mitosis. These gametes fuse during fertilization and start the sporophyte generation that will give rise to the haploid spore after meiosis. Spores are able to develop into a multicellular organism without fusing with an other cell while gametes need to go through fertilization to develop into a multicellular organism.

c. walled spores produced in sporangia (a capsule in which meiosis occurs and haploid spores form). Spores have tough walls that resist various environmental conditions and make it possible to spread them through dry air without harm. This type of spore is a major adaptation to dry land.

d. multicellular gametangia (multicellular organs in which gametes are formed). The female gametangium is called archegonium, each produces a single egg. The male gametangium is called antheridium that produces and releases sperm into the environment.

e. multicellular, dependent embryos – These embryos develop from the zygote and are retained within the female parent. The embryo takes nutrients from the mother plant. This is a very important adaptation of plants to dry land.

Other adaptations of land plants to dry land:a. cuticle – the epidermis, outer tissue of many plants is

covered with cuticle that is a waxy waterproof layer that protects the plant from water loss and from microbes.

b. Secondary compounds – these compounds are produced by secondary metabolic pathways. They protect the plant from herbivores, parasites, UV radiation etc.

Plants are classified into 10 different phyla. Plants are also informally classified by the following characteristics:

a. Vascular tissue – most plants have a set of cells that joined into tubes to transport water and nutrients throughout the body. These plants are called vascular plants. While the ones without the vascular tissue are called bryophytes (nonvascular plants) that include liverworts, hornworts and mosses. These plants also lack true leaves and roots.

b. Seedless vascular plants – most plants also have seeds, but there are two groups of plants called lycophytes (club mosses) and pterophytes (ferns) do not form seeds.

c. Seed plants – seed is an embryo packaged with a supply of nutrients inside a protective coat. Seed plants are divided into two subgroups:o Gymnosperms – these plants have seeds that are not

enclosed in chambers.o Angiosperms – these plants have seeds that develop

inside chambers called ovaries which originate within flowers and mature into fruits

III. CHARACTERISTICS OF BRYOPHYTES: Bryophyte and Bryophyta do not mean the same thing. Bryophyte

includes all nonvascular plants (liverworts, hornworts, mosses) while Bryophyta includes only mosses.

A. Gametophyte Generation: This is the dominant generation of bryophytes. It lives longer and it is

larger in size than the sporophyte generation. The gametophyte generation starts with spores that germinate if dispersed into a favorable environment. First these germinating spores form thin filament-like structures. These will provide large surface area for water

and mineral absorption. If conditions are favorable, the filaments produce “buds” that form gamete producing structures. These structures form the main body of the gametophyte generation. These gametophytes are generally thin and lack vascular tissue – short plants. The gametophytes are anchored by rhizoids – these are only a few cell thick structures that do not participate in water and mineral absorption. In most mosses the female and male gametangia are located on separate plants and the sperm cells require water to swim to the egg cell for fertilization. After fertilization, the embryo remains in the archegonium. These embryos develop into the sporophyte generation.

B. Sporophyte Generation: Bryophyte sporophytes do not live independently. They remain

attached to the parental gametophytes and absorb sugars, amino acids, minerals and water from it.

C. Ecological and Economic Importance of Mosses: They are common in moist forests and wetlands all over the world.

Many species can dry out in dry environments and than rehydrate themselves when it is rainy.

Wetland moss (peat moss) can form peat bogs that are good sources of fossils because they slow down decay and preserve dead organisms. They also store carbon (carbon reservoir) that can help with stabilizing CO2 levels and slow global warming. Also can be used as a fuel source.

IV. CHARACTERISTICS OF SEEDLESS VASCULAR PLANTSA. Origins and Traits: They have flagellated sperm that has to swim in water to the

archegonium. For that reason, these organisms live in damp areas. Sporophytes are not dependent on gametophytes but independent

organisms. The sporophyte and gametophyte generation have about

equal life span. This way more spores can be produced, greatly increasing the chance of survival. The gametophytes typically are tiny structures that grow just below the surface.

Vascular plants have two types of vascular tissue: xylem and phloem. Xylem conducts most of the water and minerals from the roots to the

leaves. The cells are called tracheids that conduct the water and minerals. These are dead cells that have their walls and act as microscopic pipes. The cell walls are lignified (contain lignin) to make the pipes stronger.

Phloem includes living cells that conduct sugars and other organic materials from the leaves or other sugar storage areas to the rest of the plant.

Xylem and phloem cells also provide strength to the stem of plants and allowed them to grow taller than bryophytes.

Roots also evolved in almost all vascular plants. The root tissue also contains lignified transport tissue which provides better support to the plant. Roots also enable vascular plants to absorb water and nutrients from the soil. Because root tissue structure resembles early plants’ shoot structure, it is likely that roots evolved from stems in ancient vascular plants.

Leaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy for photosynthesis.

B. Sporophylls and Spore Variations An important evolutionary milestone is the emergence of sporophylls –

modified leaves that bear sporangia. In higher plants sporophylls form parts of flowers.

In ferns sporophylls produce clusters of sporangia called sori, while in gymnosperms the groups of sporophylls form cones.

Most seedless vascular plant is homosporous – one type of sporophyll produces one type of spore that develops into a bisexual gametophyte generation (most ferns).

All seed plants are heterosporous – have two types of sporophylls and produce two kinds of spores. Megasporophylls produce megasporangia and in them megaspores are produced. Megaspores develop into the female gametophyte generation.

Microsporophylls produce microsporangia that give rise to microspores. Microspores develop into the male gametophyte generation.

C. Classification of Seedless Vascular Plants: Living seedless vascular plants form two clades: lycophytes (include club

mosses, spike mosses, quillworts) and pterophytes (ferns, horsetails, whisk ferns and their relatives)

Lycophytes had their evolutionary peak in the Carboniferous period when they ruled the planet. They had both small herbaceous forms and large tree-like forms. Most of those species are extinct today. The small lycophytes survived and have about 1200 species today.

Among Pterophytes ferns are the most widespread seedless vascular plants with about 12 000 species.

D. The Significance of Seedless Vascular Plants: Carboniferous forests’ lycophytes and pterophytes accelerated

photosynthesis and dramatically reduced CO2 levels in the atmosphere. This caused global cooling on the Earth and resulted in widespread glacier formation – ice age

These forests also resulted in the first coal sources that was crucial for the Industrial Revolution and is still used today – global warming

CHAPTER 30 – PLANT DIVERSITY IIOBJECTIVE QUESTIONS

Key Terrestrial Adaptations Were Crucial to the Success of Seed Plants:1. Name five terrestrial adaptations that contributed to the success of seed plants.2. Compare the size and independence of the gametophytes of bryophytes with those of seed plants.3. Describe the ovule of a seed plant.

4. Explain why pollen grains were an important adaptation for successful reproduction on land.5. Compare spores with seeds as dispersal stages in plant life cycles.Gymnosperms:6. Explain how climatic changes with the formation of the supercontinent Pangea favored the spread of gymnosperms.7. Describe the life history of a pine. Indicate which structures are part of the gametophyte generation and which are part of the sporophyte generation.Angiosperms (Flowering Plants):8. Draw a diagram of a flower and identify the following structures. Describe their function:

a. sepalb. petalc. stamend. carpele. filamentf. antherg. stigmah. stylei. ovaryj. ovule

9. Define fruit. Explain how fruits may be adapted to disperse seeds.10. Diagram the generalized life cycle of an angiosperm. Indicate which structures are part of the gametophyte generation and which are part of the sporophyte generation.11. Describe the role of the generative cell and the tube cell within the angiosperm pollen grain.12. Explain the process and function of double fertilization.13. Distinguish between monocots and eudicots (dicots).14. Explain how animals may have influenced the evolution of terrestrial plants and vice versa.Plants and Human Welfare15. Name six angiosperms that are most important in the diet of the human species.16. Describe the current threat to plant biodiversity caused by human population growth.

YOU MUST KNOW: Key adaptations to life on land unique to seed plants Evolutionary significance of seeds and pollen The role of flowers and fruits in angiosperm reproduction. The role of stamens and carpels in angiosperm reproduction.

NOTES:

I. NEW ADAPTATIONS TO DRY LAND: Gametophyte generation of seed plants is greatly reduced and

microscopic. These tiny gametophytes can develop from spores and remain within the sporangia of the parental sporophyte. This arrangement protects the female gametophyte from environmental damage (drying out, UV rays etc.) and provides nutrients for it.

Figure 30.2

Heterospory: Most seed plants are heterosporous, so they produce two different kinds of spores (megasporangia produces 1 megaspore, microsporangia produces very large numbers of microspores)

Ovule – layers of sporophyte tissue called integument surround and protect the megasporangium + megasporangium + megaspore = ovule.

Figure 30.3

Microspores develop into pollen grains. The pollen grain contains the male gametophyte of the seed plant. The pollen grain is protected by a thick coat that is carried away from the parent plant by wind or animals. Pollination – the transfer of pollen to the ovule containing part of the seed plant. Germinating pollen gives rise to a pollen tube that discharges two sperm cells into the female gametophyte. The pollen can be carried to long distances and can survive harsh conditions.

Seeds – Once a sperm fertilizes an egg, the zygote develops into the sporophyte embryo and the ovule develops into a seed – a structure that contains the developing embryo, its food supply and a protective coat that resists various environmental conditions and derived from the integument.

II. GYMNOSPERMS Gymnosperms – plants that form “naked” seeds that are not enclosed

in ovaries. The seeds are found on open modified leaves that form cones.

They have 4 phyla (cycads, ginkgo, Gnetophyta and conifers), we will focus on conifers.

The life cycle of a pine: The pine tree is the sporophyte. The sporangia are located on the scales of the cone. They have two kinds of cones for the two kinds of spores. Each tree has both kinds of cones. The smaller, soft cones contain the microsporangium and produce microspores after meiosis. The tougher cones contain the megasporangium and produce megaspores after meiosis. After fertilization the seed is formed that will be spread by wind or by animals in some rare cases. The germinating seed gives rise to the pine seedling that develops from the embryo.

http://bcs.whfreeman.com/thelifewire/content/chp30/30020.html

Figure 30.6

III. ANGIOSPERMS:A. Characteristics of Angiosperms: They are classified into a single phylum. One of the new adaptations is the appearance of flowers – a unique

angiosperm structure that is specialized for sexual reproduction. It is a set of specialized leaves that form 4 rings of floral organs. Pollination frequently takes place with the help of animals. The structure of flowers:

Figure 30.7

Fruit – seed carrying organs that form from the mature ovary, but can also carry other flower parts. Seeds develop from the ovules and the ovary thickens. The fruit protects the dormant seeds and aid their dispersal. The development of the fruit is triggered by hormonal changes after the flower is pollinated. Mature fruit can be flashy (strawberries, apples etc.) or dry (nuts, beans, grains). Various fruit adaptations help to disperse seeds such as propeller seeds of maple, burrs to cling to animals’ hair, though seed coat that resists the digestive enzymes of animals.

B. Angiosperm Life Cycle and Double Fertilization: The flower of the sporophyte produces microspores that form the male

gametophyte and megaspores that produces the female gametophyte. The male gametophyte is found in the pollen grain. Each pollen grain has two haploid cells: the generative cell that divides into two to form two sperm cells and the tube cell that forms the pollen tube. Each ovule that develops in the ovary contains a female gametophyte known as the embryo sac.

After the release from the anther the pollen is carried to the sticky surface of the stigma on the tip of the carpel. Most species cross-pollinate so the pollen of one organism usually does not fertilize the egg of the same organism. This enhances genetic variety.

The pollen grain grows a tube into the ovary of the carpel. In the ovary the pollen tube discharges two sperm cells into the female gametophyte. One sperm fertilizes the egg and forms the zygote, while the other sperm fuses with the two nuclei of the large central cell and forms the triploid endosperm which is a tissue rich in nutrients and feeds the embryo. This process is called double fertilization.

http://bcs.whfreeman.com/thelifewire/content/chp39/3902001.html After dispersal, the seed may germinate if conditions are favorable. The

seed coat ruptures and the embryo emerges as a seedling that will use the food of the endosperm and cotyledons for nutrition.

Figure 30.10

C. Angiosperm Diversity: Flowering plants are divided into several subgroups according to the

number of cotyledons (seed leaves) and some other characteristics. The two main groups are:

o Monocots – plants with one cotyledon, parallel veins, scattered vascular tissue, fibrous roots, pollen grain with one opening, floral organs with the multiples of three.

o Eudicot – plants with two cotyledons, netlike veins, vascular tissues with ring arrangement, taproot, pollen grains with three openings, floral organs with the multiples of four or five.

Figure 30.12

CHAPTER 35 – PLANT STRUCTURE, GROWTH AND DEVELOPMENT

YOU MUST KNOW: The function of xylem and phloem tissue, The specific functions of tracheids, vessels, sieve-tube elements

and companion cells. The correlation between primary growth and apical meristems

versus secondary growth and lateral meristems.

OBJECTIVE QUESTIONS:The Plant Body:1. Describe and compare the three basic organs of vascular plants. Explain how these basic organs are interdependent.2. List the basic functions of roots. Describe and compare the structures and functions of fibrous roots, taproots, root hairs and adventitious roots.3. Describe the basic structure of plant stems.4. Explain the phenomenon of apical dominance.5. Describe the structures and functions of four types of modified shoots.6. Describe the three tissue systems that make up plant organs.7. Describe and distinguish between the three basic cell types of plant tissues. For each tissue, describe one characteristic structural feature and explain its functional significance.

The Processes of Plant Growth and Development8. Distinguish between determinate and indeterminate growth. Give an example of each type of growth.9. Distinguish among annual, biennial, and perennial plants.10. Distinguish between the primary and secondary plant body.11. Describe in detail the primary growth of the tissues of roots and shoots.12. Describe in detail the secondary growth of the tissues of roots and shoots.

NOTES:

I. PLANT ORGANS Plants organs are divided into root system (below ground organs) and

shoot system (above ground organs)Figure 35.2

A. Roots: Root is an organ that anchors a vascular plant, absorbs minerals and water

and often stores organic nutrients. Most eudicots and gymnosperms have a taproot system that consists of

one main root and lateral roots. The taproot is frequently used for storage of organic nutrients until flowering uses these nutrients up (ex. Sugar beets, carrots etc.)

In monocots the root becomes fibrous root system. This root system is usually shallower that the taproot. Many monocots have sturdy rhizomes for anchoring the plant (plams, bamboo).

The entire root is important in anchoring but only the tips of the root (root tip) perform absorption of water and nutrients. The surface of the root tip is hugely enlarged by root hairs – extensions of the root epidermal cells (microscopic structures).

Nutrient absorption is also enhanced by symbiotic relationships between fungi and bacteria and the plant.

Some plant roots are modified for absorption, support or storage.Figure 35.4

B. Stems: Stem – organ consisting of an alternating system of nodes (the point at

which the leaves are attached) and internodes (segments between nodes). In the angle that is formed by each leaf and the stem is an

axillary bud (a structure that has the potential to form a lateral shoot or branch). These buds are usually dormant and the growth of the plant is concentrated on the terminal bud which is in the tip of the plant – apical dominance. By concentrating resources toward elongation, apical dominance is an evolutionary adaptation that increases the plant’s exposure to light. If the light is not appropriate or the terminal bud is damaged the auxillary buds break dormancy.

Modified stems formed in many plants. Examples of modified stems: stolons, rhizomes, tubers, bulbs.

Figure 35.5

C. Leaves Leaf – the main photosynthetic organ of most vascular plant. They

generally consist of a flattened blade and a stalk called petiole. Many monocots lack petioles and the base of the leaf forms a sheath around the stem. Veins support nutrients and take away the organic material that is produced in the leaf.

II. PLANT TISSUES: Plants are made up of three main tissue types: dermal, vascular and

ground tissues

Dermal tissue system – the outer protective covering of the plant that protects against physical damage and pathogenic organisms. In nonwoody plants it is a single layer of cells that are tightly packed and covered with cuticle – epidermis. In woody plants the periderm forms that is composed of several layers of cells.

Vascular tissue system – carries out long-distance transport between the roots and shoots (xylem and phloem). The vascular tissue of a root or a stem forms the stele. The arrangement of the stele varies depending on the organ. The stele in the root forms the vascular cylinder, while the stele in the stem forms vascular bundles.

Ground tissue system – Ground tissue that is inside the vascular tissue is called the pith, if it is outside the vascular tissue it is called the cortex.

III. CELL TYPES IN PLANTS: Figure 35.9 Parenchyma cells – have only primary cell walls that are flexible and are

able to grow. The protoplast has a large vacuole in older cells. These cells perform most of the metabolic processes of the cell. They can also retain their ability to divide and differentiate so they can be used for tissue repair.

Collenchyma cells – These cells have thicker primary cell walls but do not contain secondary cell walls. These cells provide flexible support for young stems and roots without restraining growth.

Sclerenchyma cells – These cells are equipped with secondary cell walls and usually strengthened by lignin. Mature sclerenchyma cells cannot elongate. They are specialized for support and many die when they reach maturity leaving only the cell wall behind.

Water-conducting cells of the xylem – There are two kinds of cells that build up the xylem: tracheids and vessel elements. These cells are dead at maturity and have perforated cell walls. Water can easily flow through them like on a pipe. The vessel elements are somewhat thicker and shorter than the tracheids.

Sugar-conducting cells of the phloem – These cells are alive at maturity. The sieve-tube members line up like a tube and the organic nutrients travel in the plasma of these cells. They lack most cell organelles. Companion cells are connected to the sieve-tube members by plasmodesmata. Companion cells have nuclei that may serve the sieve-tube element as well as the companion cell.

IV. PLANT GROWTHA. Meristems Plants have indeterminate growth – they grow throughout their entire

life except for some periods of dormancy. Based on the length of their life cycle, plants can be categorized as:

a. annuals – they complete their life cycle in a year or lessb. biennials – generally live for two years. The first year

they usually have their vegetative growth and the second year they flower and bear fruit or seeds

c. perennials – live for many years (the oldest living plants are thousands of years old) and they usually die of some kind of environmental stress or infection

Plants are capable of indeterminate growth because they have embryonic tissues during their entire lifetime in certain areas of their body. This tissue is called meristem. There are two main types of meristems:

o Apical meristem – located at the tips of roots and in the buds of shoots, enables the growth of plants – primary growth. In herbaceous plant primary growth produces almost all of the growth of the plant.

o Lateral meristems – found in the stem and root of the plant and called the cork cambium (replaces the epidermis in woody plants) and vascular cambium (adds layers of vascular tissue called secondary xylem and secondary phloem). The lateral meristem is responsible for secondary growth – increases the thickness of the plant.

In woody plants primary and secondary growth occurs simultaneously but in different locations.

Figure 35.10

V. PRIMARY GROWTH:A. Primary Growth in Roots: The root tip is covered by a thimble-like root cap – a protective layer

over the apical meristem. It also secretes polysaccharide slime that lubricates the soil. Growth occurs right behind the root cap in three zones:

a. Zone of cell division – includes the root tip (apical meristem) and its derivatives. New root cells are produced in this region.

b. Zone of elongation – root cells elongate and with this they push the root deeper into the soil.

c. Zone of maturation – cells complete their differentiation and become functionally mature

Figure 35.12

B. Organization of Tissues in the Young Root:You must be able to label the transverse section of a root. Figure 35.13

Water and minerals from the soil must enter through the epidermis, a single layer of cells covering the root. Root hairs enhance this process by greatly increasing the surface area.

In most roots the stele is a vascular cylinder. The xylem radiates from the center, with the phloem developing in the wedges between the spokes. In many roots the vascular tissue consists of a central core of parenchyma cells surrounded by alternating rings of xylem and phloem.

The ground tissue mostly found in the cortex that is the area between the epidermis and the vascular cylinder. The cells of the cortex store organic materials and participate in mineral absorption from the soil. The innermost layer of the cortex is the endodermis that forms a boundary with the vascular cylinder and regulates the flow of substances from the soil into the vascular cylinder

C. Primary Growth in ShootsYou must be able to label the parts of a stem in both monocots and eudicots. Figure 35.16

The shoot apical meristem is a dome shaped mass of dividing cells at the tip of the terminal bud.

The tissue organization of the stem:o Epidermis covers the stem from the outsideo Vascular tissue runs in the form of vascular bundles in the stem

that converge with the vascular cylinder of the root at the zone of transition near the soil surface. In gymnosperms and in eudicots the vascular bundles are arranged in a ring, the xylem faces the pith and the phloem faces the cortex. In monocots the bundles are scattered in the ground tissue of the stem. The ground tissue is mostly made up of parenchyma cells, but sclerenchyma cells are also found there for support (fibers)

D. Tissue Organization of the LeafYou must be able to label parts of the leaf cross section. Figure 35.17

The leaf is also covered by the epidermis that is punctured by stomata (allows gas exchange and important in water regulation). The stomata are surrounded by two guard cells, which regulate the opening and closing of the pore.

The ground tissue is sandwiched between the upper and lower epidermis. The ground tissue in the leaf is called the mesophyll that mostly consists of parenchyma cells. Palisade mesophyll is the upper part of the mesophyll with a single row of long cells. The lower part is the spongy mesophyll that is below the palisade mesophyll. The loosely arranged cells of the spongy mesophyll have air spaces that circulate CO2 and O2 around the cells.

The vascular tissue of the leaf is in the vein. The xylem is toward the top of the leaf, the phloem is toward the bottom. The veins are surrounded by bundle sheath that is one layer of parenchyma cells – C4 plants.

VI. SECONDARY GROWTH IN PLANTS Secondary growth adds thickness to the plant body. It forms the cells

that are produced from the vascular cambium or the cork cambium of the stem. The vascular cambium adds secondary xylem and secondary phloem (becomes part of the bark), while the cork cambium adds a tough, thick covering (bark of trees).

Figure 35.18

We will not cover anything from Section 35.5 and you will not be tested on it either.

CHAPTER 36 – TRANSPORT IN VASCULAR PLANTSYOU MUST KNOW:

The role of active, passive and cotransport in plant transport. The role of diffusion, active transport, and bulk transport in the

movement of water and nutrients in plants. How the transpiration cohesion-tension mechanism explains

water movement in plants How pressure flow explains translocation.

OBJECTIVE QUESTIONS:An Overview of Transport Mechanisms in Plants1. Describe how proton pumps function in transport of materials across plant membranes, using the terms proton gradient, membrane potential, cotransport, and chemiosmosis.2. Define osmosis and water potential. Explain how water potential is measured.3. Explain how solutes and pressure affect water potential.4. Explain how the physical properties of plant cells are changed when the plant is placed into solutions that have higher, lower or the same solute concentration.5. Define the terms flaccid, plasmolyze, turgor pressure, and turgid.6. Explain how aquaporings affect the rate of water transport across membranes.7. Distinguish between the symplast and apoplast.8. Describe three routes available for lateral transport in plants.9. Define bulk flow and describe the forces that generate pressure in the vascular tissue of plants.10. Relate the structure of sieve-tube cells, vessel cells and tracheids to their functions in bulk flow.

Absorption of Water and Minerals by Roots11. Explain what routes are available to water and minerals moving into the vascular cylinder of the root.12. Explain how mycorrhizae enhance uptake of materials by roots.13. Explain how the endodermis functions as a selective barrier between the root cortex and vascular cylinder.

Transport of Xylem Sap14. Describe the potential and limits of root pressure to move xylem sap.15. Define the terms transpiration and guttation.16. Explain how transpiration pull moves xylem sap up from the root tips to the leaves.17. Explain this statement: “The ascent of xylem sap is ultimately solar powered.”

The Control of Transpiration18. Explain the importance and costs of the extensive inner surface area of a leaf.19. Discuss the factors that may alter the stomatal density of a leaf.20. Describe the role of guard cells in photosynthesis-transpiration.21. Explain how and when stomata open and close. Describe the cues that trigger stomatal opening at dawn.22. Explain how xerophytes reduce transpiration.

Translocation of Phloem Sap

23. Define and describe the process of translocation. Trace the path of phloem sap from a primary sugar source to a sugar sink.24. Describe the process of sugar loading and unloading.25. Define pressure flow. Explain the significance of this process in angiosperms.

NOTES:

I. PHYSICAL FORCES THAT DRIVE THE TRANSPORT OF MATERIALS IN PLANTS Transport of materials occurs on three scales in plants:

o Transport of water and solutes by individual cellso Short-distance transport of substances from cell to cell on the tissue

levelo Long-distance transport within the xylem and phloem cells at the level of

the whole plantReview transport processes in Chapter 7 The most important active transport process in the plasma membrane of

plants is the proton pump, which uses energy from ATP to pump H+ ions out of the cell. This results in a higher H+ concentration outside of the cell than inside. This concentration gradient is a form of potential energy because the hydrogen ions can be used to do work while diffusing back into the cell. Because of the charge difference between the two sides of the cell membrane (and wall) the pump also contributes to a voltage known as membrane potential. This potential energy and membrane potential is used to transport various solutes (such as K+ ions).

In cotransport a transport protein couples the downhill passage of a solute (H+) to the uphill transport of another molecule. This transport mechanism is responsible for the uptake of NO3

- ions (important in building amino and nucleic acids) and the intake of sugar by plant cells. Chemiosmosis also uses this process.

Figure 36.4

A. Water Potential Plants must balance water uptake and water loss to survive. Water moves

by osmosis in and out of the cells. In plants it is more complicated to predict which direction water is moving

to because of the presence of the cell wall. In the case of plants pressure and solute concentration together determines the direction of water movement. The combined measure of solute concentration and physical pressure are incorporated into water potential (abbreviated with the Greek letter psi, Ψ). Water potential determines the direction of movement of water.

Free water that is not bound by solute or by surfaces always moves from the higher water potential region to the lower water potential region if there is no barrier to its flow.

The unit of water potential is megapascals (MPa). Pure water has a water potential of zero if the container is open to the atmosphere, under standard conditions.

The equation for water potential shows both of its components:Ψ = Ψs + Ψp

Where Ψs is the solute potential and ΨP is the pressure potential. Solute potential of a solution is proportional to the number of dissolved

solute molecules. Because solutes bind to water molecules and lower the capacity of water molecules to do work, adding solute molecules to water always lowers water potential. Because pure water has a zero water potential, solutions always have a negative solute potential.

Pressure potential is the physical pressure on a solution. Pressure potential can be both positive and negative relative to atmospheric pressure. The pressure potential in living cells is usually under positive pressure. This positive pressure that presses the plasma membrane against the cell wall is called turgor pressure.

B. Analysis of Water Potential in an Artificial SystemFigure 36.5

Part a: The membrane that separates the two sides of the U-tube is

permeable to water but not to solutes. If one side is filled with pure water and the other side is filled with a 0.1M solution, the net water movement will be toward the solution from the pure water, because water always

moves from higher water potential to the lower water potential area.

Part b: If a pressure of 0.23 MPa is applied to the solution that the water potential is raised to 0 MPa. There will be no net flow between the two sides.

Part c: If the pressure is further increased to 0.3 MPa than the solution has a water potential of +0.07 MPa. This solution will actually lose water to the pure water part of the U-tube.

Part d: If the plunger would be pulled back on the pure water side, it would create a negative pressure and water would move to the pure water side.

In plant cells, if a plant cell is limp (flaccid) it would have a ΨP of 0 and if it is

sitting in a more concentrated environment, the cell would lose water and would plasmolyze. However, if the plant cell is in pure water, it would gain water and become turgid until the cell wall would produce enough pressure to compensate for the concentration difference and Ψ = 0.

Turgor pressure is an important tool of plant support in nonwoody plants.

C. Major Compartments of Vacuolated Plant Cells: The cell wall helps to maintain the cell’s shape The plasma membrane is a barrier between the cell wall and the cytosol. Mature plant cells also have vacuoles that are surrounded by the vacuolar

membrane (tonoplast) – regulates molecular traffic between the cytosol and the vacuolar contents (cell sap). Proton pumps move hydrogen ions into the vacuole and other ions will follow the H+ ions by cotransport (chemiosmosis).

In most cells the cytoplasm and the cell wall is continuous from cell to cell. The cytoplasmic continuum is formed by plasmodesmata and is called symplast. The continuum formed by the cell wall is called apoplast. The vacuole is independent from the other cells.

Figure 36.8

D. Short-distance Transport Within the plant tissues, water and mineral can move from cell to cell (from

the root hair to the vascular tissue) in three ways:o Apoplastic route – substances move across the cell wall from one

cell to the next. o Symplastic route – substances can move on the cytoplasmic route,

so they have to cross only one cell membrane and than flow from one cell to the next via the cytoplasm.

o Transmembrane route – substances can move across the cell wall and cell membrane of one cell than the next one, so the plasma membrane needs to be crossed repeatedly.

E. Long-Distance Transport Bulk flow – is the movement of a fluid driven by pressure Diffusion is not efficient enough to perform bulk transport so other forces

act in these cases – see later in this chapter.

II. ABSORPTION OF WATER AND MINERALS FROM THE SOILFigure 36.9

Mineral ions from the soil can get into the root of plants by three different ways:

o Diffusion – if the concentration of certain ions is lower inside of the root hair cells, they can simply diffuse into the root hair cells from the soil

o With the help of fungal hyphae – Some plants live in a symbiotic relationship with fungi and use fungal hyphae to increase the surface of the root even more. This combination of plant root and fungal fibers are called mycorrhiza. The fungus benefit from a constant supply of sugar (organic nutrients) while the plant benefit from the increased surface area that the fungal hyphae provide, they also excrete growth factors and antibiotics.

o The flow of water into the root can also carry ions passively in dissolved form.

Root hairs expend the surface area of roots. These roots hairs that are extensions of single epidermal cells absorb water and dissolved minerals.

Watery solution has to be transported from the soil to epidermal cells to cortex of the root to the xylem (water-conducting vascular tissue)

The plasma membrane of the xylem cells are selectively permeable (only certain materials can go through them)

There are 2 possible routes to the xylem:o Intracellular route (symplastic route): Via cell interiors through

plasmodesmata (continuum of cytoplasm between cells), so crossing cell walls and cell membranes is only once necessary.

o Extracellular route (apoplastic route): The solution moves within the hydrophilic walls and extracellular spaces but does not enter the cytoplasm of the cortex or epidermis cells. The Casparian strip (a waxy layer of the endodermis) stops these substances from entering the xylem, so they need to travel through the endodermal cell by the intracellular route.

o In real life solutions are transported by a mix of these two routes.

III. LONG TERM TRANSPORT OF WATER AND MINERALS The transport takes place in the xylem and moves substances from the

roots to the shoot system to the veins of each leaf. Plants lose water by transpiration – the loss of water vapor from leaves

and other aerial parts of the plant. The transpired water is replaced by water that is absorbed by the roots.

Two forces contribute to water transport:a. Root pressure – Water flows in from the root cortex and

pushes the water that is already there upwards, toward the leaves. This water flow is due to the accumulation of minerals in the vascular cylinder that lowers water potential there and pulls water in. In smaller plants this pushing force of water results in guttation – the collection of water droplets on the leaves early in the morning. This force is especially important at night when transpiration is very low or zero because of the low temperature. But this force is too small to push water up higher than a few meters.

b. Transpiration-Cohesion-Tension Mechanism -- a pulling force that is created by the loss of water from the leaves and other aerial parts of the plant. Water molecules leave the plant through the stoma of the leaf by diffusion because when the stoma is open, water concentration is higher in the plant cells than in the surrounding atmosphere. Because water molecules are polar, they are attracted to each other by hydrogen bond (cohesion). These water molecules form continuous strings of water

from the root to the leaves. In the meantime, adhesion sticks molecules and ions of different kinds together. This force adheres water molecules to cellulose molecules in the xylem cell wall. As water molecules transpire away from one end of the string of water molecules because of the concentration gradient, cohesion and adhesion pulls the remaining water molecules upwards from the root. These processes do not require energy from the plant, they are all extended by physical properties of water and the surrounding molecules.

Figure 36.13

Plant transpiration site (great) : http://croptechnology.unl.edu/animation/transpiration.swf

IV. STOMATA AND TRANSPIRATION Stomata are used to take in CO2 from the environment. The high

surface-to-volume ratio of leaves help to take in more CO2 for

photosynthesis. However, this process enhances water loss by transpiration.

Opening and closing stomata somewhat lowers the rate of evaporation but even when the stomata are closed, they still evaporate water.

Factors increasing the rate of transpiration: higher temperature, wind, dry air.

Factors decreasing the rate of transpiration: lower temperature, cuticle, humidity

A. Mechanism of stomatal opening and closing: Opening: guard cells take in water from neighboring cells by osmosis,

they become turgid and bowed. Guard cells can take in water because they accumulate K ions by active transport from neighboring cells – negative water potential inside the guard cells. The negative water potential will pull water in from the surrounding cells. The energy for the active transport is provided by proton pumps and the cotransport of H ions.

Closing the stomata: guard cells lose K ions to the neighboring epidermal cells and water will passively follow the potassium ions because of the positive water potential that in created inside the guard cells.

Stomata can also be opened if CO2 is depleted from the air spaces under the stomata.

Circadian rhythms – Stomata are open in a regular rhythm during the day and closed at night.

Environmental stresses can cause the stomata to close during the day (water deficiency)

Figure 36.15

B. Xerophyte adaptations: Xerophytes – plants that adapt to dry conditions There are various leaf modifications that help plants to survive dry

conditions (thick cuticle, small leaves, hair on the leaves, depressions for the stomata)

The C4 and CAM photosynthetic processes are also adaptations

V. MOVEMENT FROM SUGAR SOURCES TO SUGAR SINKS (MOVEMENT OF ORGANIC SUBSTANCES)

Organic materials are transported in the sieve-tube members of the phloem.

Sucrose is the most important organic material that is transported in the phloem but amino acids, minerals and hormones are also transported in the phloem sap.

The direction in which the phloem sap travels varies depending on the location of the organic material. Sugar source – is the plant organ that is a net producer of sugar (leaf, storage root etc.) A sugar sink – is a consumer of the sugar that needs to receive sugar from the source (growing buds, stems, fruits). The sugar sink usually receives sugar from the nearest source.

Sugar must be loaded into the sieve tube members before it can be transported. It usually moves through the plasmodesmata of neighboring cells via the symplast, but in some plants they also use the apoplast route. Cotransport with H ions move the sugar into the sieve tube members of the phloem. The sugar concentration is always higher at the sugar source than at the sink so it will move in the phloem from the high to the lower sugar concentration area. Water follows by osmosis. The movement is sped up by positive pressure that is created by water from the xylem. The high pressure will press the water with the sugar toward the sugar sink where the water exits back to the xylem. This process that enhances sugar flow is called pressure flow.

Figure 36.18

http://academic.kellogg.cc.mi.us/herbrandsonc/bio111/animations/0032.swf http://bcs.whfreeman.com/thelifewire/content/chp36/36020.html